spi: dt-bindings: Convert spi-gpio binding to json-schema

Convert the spi-gpio binding to DT schema format.

Cc: Mark Brown <broonie@kernel.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-spi@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>